AWWA WQTC71500

AWWA WQTC71500 Seasonal Direct Filtration to Reduce Water Treatment Plant Residuals
Conference Proceeding by American Water Works Association, 11/01/2009

James, Wendell; Molla, Saifuddin; Craik, Stephen; Gammie, Les; Thomas, Simon; Shariff, Riyaz

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EPCOR Water Services Inc. is exploring an innovative strategy to reduce treatment residuals andcoagulant cost. The utility operates two water treatment plants supplied by the NorthSaskatchewan River in Edmonton, Alberta, Canada. Both plants normally operate in coagulation-sedimentation-filtration mode with alum as the primary coagulant. Challenging spring and summer conditions preclude year-round direct filtration, but EPCOR initiated tests to investigate the feasibility of operating plants in direct filtration mode during the winter. Converting to direct filtration mode would substantially reduce alum use and the associated alum sludge production. Savings would be realized due to lower chemical costs, less sludge handling, and reduced clarifier maintenance. The objective of this study was to determine whether direct filtration could be implemented during winter while meeting treatment objectives for removing turbidity, particle counts and microorganisms. Bench-scale tests showed that alum doses of 4 to 5 mg/L followed by cationic filter aid doses between 0.2 and 1.0 mg/L were adequate to remove 75% of the turbidity and particles greater than 2 microns within 5 minutes of filtration time. Pilot trials that included pre-filter chlorination and an alum dose of 5 mg/L produced filtered water quality consistently meeting the following criteria: turbidity 0.1 NTU and particle counts 20/mL. These criteria could be met with or without using flocculant and filter aids. A two-day demonstration conducted at the Rossdale Water Treatment Plant (WTP) compared direct filtration to conventional operation. Direct filtration using 5 mg/L of alum and 0.4 mg/L of filter aid polymer allowed water quality objectives to be met consistently, without substantially affecting filter run times. For an average winter water demand of 327 ML/day (both Edmonton WTPs), the cost saving related to reduced chemical use would be $2500/day. Includes 5 references, tables, figures.